Luffy would lose his power, because Haki does not seem to negate the power of the sea at all. In chapter 603, we could see how Luffy put his hand into the ocean without coating it in Haki. He said to be powerless in the ocean, yet he was able to draw Sanji back to the boat. This would make it seem that due to his training during the time-skip, he has become able to withstand the power of the sea for a short while.
Then, in chapter 605, we could see how Luffy put his hand back into the ocean, but this time he coated it in Haki. Despite being fully coated, he still lost his power at the same rate. Luffy got a lot stronger during the time-skip and it might even be that, since he has more power to drain, it might take a bit longer, but it is safe to say that coating yourself in Haki does not negate the effects of the sea.